1. Field of the Invention
This invention is related to the practice of sidetrack drilling for hydrocarbons. More specifically, this invention pertains to a whipstock assembly for creating a window within a wellbore casing. More particularly still, the invention pertains to a whipstock that more easily permits penetration of perforation shots through the perforation plate.
2. Description of the Related Art
In recent years, technology has been developed which allows an operator to drill a primary vertical well, and then continue drilling an angled lateral borehole off of that vertical well at a chosen depth. Generally, the vertical, or “parent” wellbore is first drilled and then supported with strings of casing. The strings of casing are cemented into the formation by the extrusion of cement into the annular regions between the strings of casing and the surrounding formation. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
In many instances, the parent wellbore is completed at a first depth, and is produced for a given period of time. Production may be obtained from various zones by perforating the casing string. At a later time, it may be desirable to drill a new “sidetrack” wellbore utilizing the casing of the parent wellbore. In this instance, a tool known as a whipstock is positioned in the casing at the depth where deflection is desired, typically at or above one or more producing zones. The whipstock is specially configured to divert milling bits into a side of the casing in order create an elongated elliptical window in the parent casing. Thereafter, a drill bit is run into the parent wellbore. The drill bit is deflected against the whipstock, and urged through the newly formed window. From there, the drill bit contacts the rock formation in order to form a new lateral hole in a desired direction. This process is sometimes referred to as sidetrack drilling.
When forming the window through the casing, an anchor is first set in the parent wellbore at a desired depth. The anchor is typically a packer having slips and seals. The anchor tool acts as a fixed body against which tools above it may be urged to activate different tool functions. The anchor tool typically has a key or other orientation-indicating member. The anchor tool's orientation is checked by running a tool such as a gyroscope indicator or measuring-while-drilling device into the wellbore.
A whipstock is next run into the wellbore. The whipstock has a body that lands into or onto the anchor. A stinger is located at the bottom of the whipstock which engages the anchor device. In this respect, splined connections between the stinger and the anchor facilitate correct stinger orientation. At a top end of the body, the whipstock includes a deflection portion having a concave face. The stinger at the bottom of the whipstock body allows the concave face of the whipstock to be properly oriented so as to direct the milling operation. The deflection portion receives the milling bits as they are urged downhole. In this way, the respective milling bits are directed against the surrounding tubular casing for cutting the window.
In order to form the window, a milling bit, or “mill,” is placed at the end of a string of drill pipe or other working string. In one arrangement, the mill includes cutting blades that are spiraled in order to form water courses there between. An alloy of nickel and crushed carbide is typically placed at the tip of the mill for frictionally engaging the steel casing as the mill bit is rotated. In the usual milling operation, a series of mills is run into the hole. First, a starting mill is run into the hole. Rotation of the string with the starting mill rotates the mill, causing a portion of the casing to be removed. This mill is followed by other mills, which complete the creation of the elongated window.
FIG. 1 presents a cross-sectional view of a wellbore 10. As completed in FIG. 1, the wellbore 10 has a first string of surface casing (not shown) hung from the surface. The first string is fixed in a formation 20 by cured cement. A second string of casing 30 is also present in the completed wellbore 10. The second casing string 30, sometimes referred to as a “liner,” is hung from the surface casing by a conventional liner hanger (not shown). The liner hanger employs slips which engage the inner surface of the surface casing to form a frictional connection. The liner 30 is also cemented into the wellbore 10 after being hung from the surface casing. A column of cured cement 35 is shown in FIG. 1 in the annular region between the liner 30 and the surrounding formation 20.
The wellbore 10 of FIG. 1 includes a working string 50 that is run into the hole. Attached to the working string 50 at the lower end is a mill 60. The mill 60 is shown somewhat schematically. It is understood that the initial mill 60, referred to as a “starter” mill, is more elongated and frequently employs more than one set of cutting blades, as will be described in connection with FIG. 3. Rotation of the working string 50 imparts rotary movement to the starter mill 60.
FIG. 1 also presents, somewhat schematically, a side view of a whipstock 80. The whipstock 80 is known in the art. A fuller, cross-sectional view of a prior art whipstock 80 is shown in FIG. 2. The whipstock 80 has a top end that is releasably connected to a pilot lug 70 by shear studs 75. The pilot lug 70 serves as a sacrificial element in the initial cutting of a window. It is understood that the pilot lug 70 is an optional feature, but is nevertheless commonly used.
The whipstock 80 has a body 120 that defines an outer metal shell and an inner cavity 150. The body 120 of the whipstock 80 has a bottom end 122 that lands upon an anchor. The anchor is shown at 90 in FIG. 1. It can be seen in FIG. 1 that the anchor 90 may be a packer having centralizers 92, slips 94, and a sealing element 96. The bottom end 122 of the whipstock 80 includes an orientation key 130. The orientation key 130 lands in the anchor 90 and aids in properly orienting the whipstock 80 downhole.
The whipstock 80 also comprises a deflection portion 170. The deflection portion 170 of the whipstock 80 is at the top end of the whipstock 80, and serves to urge the mill 60 outwardly against the surrounding tubular 30, e.g. casing, during a milling operation. The deflection portion 170 typically defines a concave-shaped portion of the body 120 that serves as a concave-shaped member 111. In the case of a perforation whipstock 80, the concave-shaped member 111 includes a plate referred to as a “perforation plate” 110. As will be set forth in detail below, the perforation plate 110 receives shaped charges (or other perforation explosives) during subsequent wellbore completion operations. In this manner, production may again be obtained from the primary wellbore. More specifically, the operator may produce fluids from the original formation through the anchor, the packer, and then through a cavity 160 within the whipstock body.
The cavity 160 in some whipstock arrangements is partially filled with cement, and with a bore optionally retained therethrough. More recent whipstock designs retain a hollow cavity 160. In this manner, the whipstock body serves as a pressure-retaining vessel until perforations are placed in the perforation plate 110. However, in prior art whipstock designs, the perforation plate 110 has a limited pressure capacity, i.e., burst pressure, because the perforation plate 110 simply represents a plate welded onto a formed ramp in the whipstock body. As will be discussed further below, a need has existed for a whipstock assembly having a greater burst pressure capacity.
As noted above, a mill 60 is run into the wellbore 10 in order to begin milling a window in the casing string 30. An exemplary starting mill 200 is shown in FIG. 3. The starting mill 200 has a body 202 with a fluid flow channel 204 therethrough (shown in dotted lines). Three sets of cutting blades 210, 220, and 230 with, respectively, a plurality of blades 211, 221, and 231 are spaced apart on the body 202. Jet ports 239 are in fluid communication with the channel 204.
The exemplary starting mill 200 has a tapered nose 240 that projects down from the body 202. The mill 200 also has a tapered end 241, a tapered ramped portion 242, a tapered portion 243, and a cylindrical portion 244. It is understood that the mill 200 in FIG. 3 is exemplary only; the present invention is not limited in scope by the type of starter mill employed, or the manner in which it is run into a wellbore 10.
The starter mill 200 is slowly lowered to contact the pilot lug 70 (or some sacrificial element) on the concave-shaped member 111 of the whipstock 80. The starter mill 200 moves downwardly while contacting the perforation plate 110 of the whipstock 80. This urges the starting mill 200 into contact with the casing 30. As the mill 200 initially moves down in the wellbore, the blades 230 begin to mill the pilot lug 70 and any other sacrificial element, e.g., nose 240. The pilot lug 70 and any other sacrificial element are chewed by the lower starter blades 230. As the starter mill 200 moves further downwardly, the lower blades 230 contact the perforation plate 110 of the whipstock 80. The angled geometry of the concave-shaped member 111 of the whipstock 80 urges the starter blades 230 outwardly into contact with the adjacent casing 30. These lowest blades 231 then begin milling into the casing 30 to form the initial window at the desired location. The casing 30 is milled as the pilot lug 70 is milled off.
Milling of the casing 30 is achieved by rotating the tool 200 against the inner wall of the casing 30 while at the same time exerting a downward force on the drill string 50 against the whipstock 100. After the mill 20 has moved downwardly to cause the lower blades 231 to begin milling the casing 30, the middle 221 and upper 211 blades also begin to mill portions of adjacent casing 30 above the lower blades 231. The upper blades 221, 211 are preferably configured to cut successively larger window portions. Ultimately, the starting mill 200 cuts an elongated initial window (not shown) in the casing 30. The starting mill 200 is then removed from the wellbore 10.
A window mill is next lowered into the wellbore 10. FIG. 4 presents an exemplary window mill 250 for use to enlarge the starting window made by the starter mill 200. The window mill 250 has a body 252 with a fluid flow channel 254 from top to bottom and jet ports 255 to assist in the removal of cuttings and debris. A plurality of blades 256 present a smooth finished surface 258 that move along what is left of the sacrificial element (e.g. one, two, three up to about twelve to fourteen inches) and then on the edges of the concave-shaped member 111. Lower ends of the blades 256 and even a lower portion of the body 252 are dressed with milling material 260, such as tungsten carbide chunks in a nickel alloy. The spacing between the cutting blades 256 is known as the watercourses. The watercourses permit the recirculation of fluids with suspended metal cuttings back up the wellbore 10 during the milling operation.
In one aspect, the lower end of the body 252 tapers inwardly at an angle “c” to inhibit the window mill lower end from directly contacting and milling the perforation plate 110 of the whipstock body 120. In this respect, the angle “c” is preferably greater than the angle “a” of the concave-shaped member 111, shown in FIG. 2. Preferably, the angle “a” of the whipstock 250 is three degrees. Therefore, the angle “c” for the lower ends of the blades 256 is greater than three degrees.
In one aspect, the surface 258 is about fourteen inches long and, when used with the mill 200 having blades 211, 221, 231 about two feet apart as described above, an opening of about five feet in length is formed in the casing 30 when the sacrificial element has been completely milled down. In this embodiment, the window mill 250 is then used to mill down another ten to fifteen feet so that a completed opening of fifteen to twenty feet is formed, which includes a window in the casing 30 of about eleven to fifteen feet and a milled bore into the formation adjacent the casing 30 of about five to nine feet.
The window mill 250 is lowered into the wellbore on a working string. An example is a flexible joint of drill pipe (not shown).
Additional information concerning the construction of window mills, in at least one embodiment, is found in U.S. Pat. No. 5,787,978, issued to Carter, et al. in 1998. The assignee of that patent is Weatherford/Lamb, Inc.
As a next step, the working string 50 is tripped. A drill bit 40 is then run on drill string 78 which is deflected by the whipstock 80 through the freshly milled window W. This stage of the milling operation is depicted in the view of FIG. 5. FIG. 5 presents a cross-sectional view of the wellbore 10 of FIG. 1, with a window W having been formed in the casing 30. A lateral borehole L is now being drilled, as shown by arrow 42. A drill bit 40 is shown at the end of a drilling string 78. The drill bit 40 engages the formation 20 so as to directionally form the lateral borehole L adjacent the window W. In the exemplary operation of FIG. 5, the drill bit 40 is rotated by means of a downhole rotary motor 45.
After the lateral borehole L is formed, a liner (not shown) is run into the newly formed lateral wellbore L. The liner is hung from the parent wellbore casing 30, and then cemented in place.
In some lateral wellbore completions, a perforating gun is deployed in the parent wellbore 10 as well. In this respect, it is sometimes desirable to re-establish fluid communication within the parent wellbore with a producing zone at or below the depth of the whipstock 80. In such an instance, a perforating gun (not shown) is lowered into the liner for the lateral wellbore L. The perforating gun is lowered to the depth of the whipstock 80, and fired in the direction of the whipstock's deflection portion 170. This serves to create perforations through the perforation plate 110 and the liner of the lateral wellbore L (not shown). This, in turn, re-establishes fluid communication between the surface and the original producing formation of the parent wellbore.
Various explosive perforation devices are known, including but not limited to: a jet charge, linear jet charge, explosively formed penetrator, multiple explosively formed penetrator, or any combination thereof to preferably form a shaped charge. The presence of perforations in the perforation plate 110 allows valuable production fluids to migrate up the parent wellbore 10 from producing zones at or below the level of the whipstock 80. Production fluids flow through the anchor, the packer, the cavity in the whipstock body, and through the perforation plate. From there, fluids travel up the wellbore where they are captured at the surface.
It is understood that the creation of perforations through the perforation plate is typically done after the lateral borehole has been completed. Thus, charges must be of sufficient power to penetrate through the liner of the lateral borehole L, the surrounding column of cured cement (not shown) between the liner and the whipstock's perforation plate, and finally the perforation plate itself. In order to aid in the perforation of the whipstock's 80 perforation plate 110, it is desirable to have a perforation plate 110 on the whipstock 80 that is of a sufficiently thin or pliable metal to permit penetration by the perforating explosives. While such a composition aids in perforation of the whipstock 80, it also reduces the durability of the whipstock 80 during the milling operation. In this respect, the process of urging mill bits 60 downward against the perforation plate 110 of a whipstock 80 causes some inevitable sacrifice of the plate 110 of the whipstock 80 and, in some instances, removes all of the plate 110. This, in turn jeopardizes the ability of the whipstock 80 to deflect the mill bits, e.g., bits 200 and 250 against the casing 30. It also inhibits the whipstock's ability to withstand pressures within the wellbore 10. Still further, the uneven face surface of the perforation plate 110 resulting from sacrifice during the milling process reduces the effectiveness of the shaped charges.
Additionally, the prior art whipstock is difficult to manufacture. In this respect, the joining of the thin perforation plate and the outer body of the perforation whipstock is difficult to fabricate and can cause failures before the additional stress of the milling operation. This further jeopardizes the ability of the whipstock to withstand pressure within the wellbore, and increases the cost of manufacture.
While the pressure face is able to carry some pressure, because of the difficult manufacture process, the pressure retaining face is only able to carry a relatively low pressure, especially in larger sizes of whipstock assemblies. With the advances in other downhole tools, the requirements for this pressure retaining device to carry more pressure have exceeded its current capacity.
What is needed, then, is a whipstock arrangement that can be reliably manufactured and substantially prevents contact between the rotating mill bits, e.g., bits 200 and 250, and the perforation plate 110, while allowing for high pressure retaining capabilities.